Re: [PATCHv0 dont apply] RFC: kvm eoi PV using shared memory

Avi Kivity
2012-04-10T14:03:25Z

On 04/10/2012 04:27 PM, Michael S. Tsirkin wrote:
> I took a stub at implementing PV EOI using shared memory.
> This should reduce the number of exits an interrupt
> causes as much as by half.
>
> A partially complete draft for both host and guest parts
> is below.
>
> The idea is simple: there's a bit, per APIC, in guest memory,
> that tells the guest that it does not need EOI.
> We set it before injecting an interrupt and clear
> before injecting a nested one. Guest tests it using
> a test and clear operation - this is necessary
> so that host can detect interrupt nesting -
> and if set, it can skip the EOI MSR.
>
> There's a new MSR to set the address of said register
> in guest memory. Otherwise not much changes:
> - Guest EOI is not required
> - ISR is automatically cleared before injection
>
> Some things are incomplete: add feature negotiation
> options, qemu support for said options.
> No testing was done beyond compiling the kernel.
>
> I would appreciate early feedback.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>
> --
>
> diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
> index d854101..8430f41 100644
> --- a/arch/x86/include/asm/apic.h
> +++ b/arch/x86/include/asm/apic.h
> @@ -457,8 +457,13 @@ static inline u32 safe_apic_wait_icr_idle(void) { return 0; }
>
> #endif /* CONFIG_X86_LOCAL_APIC */
>
> +DECLARE_EARLY_PER_CPU(unsigned long, apic_eoi);
> +
> static inline void ack_APIC_irq(void)
> {
> + if (__test_and_clear_bit(0, &__get_cpu_var(apic_eoi)))
> + return;
> +

While __test_and_clear_bit() is implemented in a single instruction,
it's not required to be. Better have the instruction there explicitly.

> /*
> * ack_APIC_irq() actually gets compiled as a single instruction
> * ... yummie.
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index e216ba0..0ee1472 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -481,6 +481,12 @@ struct kvm_vcpu_arch {
> u64 length;
> u64 status;
> } osvw;
> +
> + struct {
> + u64 msr_val;
> + struct gfn_to_hva_cache data;
> + int vector;
> + } eoi;
> };

Needs to be cleared on INIT.

>
>
> @@ -307,6 +308,9 @@ void __cpuinit kvm_guest_cpu_init(void)
> smp_processor_id());
> }
>
> + wrmsrl(MSR_KVM_EOI_EN, __pa(this_cpu_ptr(apic_eoi)) |
> + MSR_KVM_EOI_ENABLED);
> +

Clear on kexec.

> if (has_steal_clock)
> kvm_register_steal_time();
> }
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 8584322..9e38e12 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -265,7 +265,61 @@ int kvm_apic_set_irq(struct kvm_vcpu *vcpu, struct kvm_lapic_irq *irq)
> irq->level, irq->trig_mode);
> }
>
> -static inline int apic_find_highest_isr(struct kvm_lapic *apic)
> +static int eoi_put_user(struct kvm_vcpu *vcpu, u32 val)
> +{
> +
> + return kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.eoi.data, &val,
> + sizeof(val));
> +}
> +
> +static int eoi_get_user(struct kvm_vcpu *vcpu, u32 *val)
> +{
> +
> + return kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.eoi.data, val,
> + sizeof(*val));
> +}
> +
> +static inline bool eoi_enabled(struct kvm_vcpu *vcpu)
> +{
> + return (vcpu->arch.eoi.msr_val & MSR_KVM_EOI_ENABLED);
> +}
> +
> +static int eoi_get_pending_vector(struct kvm_vcpu *vcpu)
> +{
> + u32 val;
> + if (eoi_get_user(vcpu, &val) < 0)
> + apic_debug("Can't read EOI MSR value: 0x%llx\n",
> + (unsigned long long)vcpi->arch.eoi.msr_val);
> + if (!(val & 0x1))
> + vcpu->arch.eoi.vector = -1;
> + return vcpu->arch.eoi.vector;
> +}
> +
> +static void eoi_set_pending_vector(struct kvm_vcpu *vcpu, int vector)
> +{
> + BUG_ON(vcpu->arch.eoi.vector != -1);
> + if (eoi_put_user(vcpu, 0x1) < 0) {
> + apic_debug("Can't set EOI MSR value: 0x%llx\n",
> + (unsigned long long)vcpi->arch.eoi.msr_val);
> + return;
> + }
> + vcpu->arch.eoi.vector = vector;
> +}
> +
> +static int eoi_clr_pending_vector(struct kvm_vcpu *vcpu)
> +{
> + int vector;
> + vector = vcpu->arch.eoi.vector;
> + if (vector != -1 && eoi_put_user(vcpu, 0x0) < 0) {
> + apic_debug("Can't clear EOI MSR value: 0x%llx\n",
> + (unsigned long long)vcpi->arch.eoi.msr_val);
> + return -1;
> + }
> + vcpu->arch.eoi.vector = -1;
> + return vector;
> +}

> +
> +static inline int __apic_find_highest_isr(struct kvm_lapic *apic)
> {
> int result;
>
> @@ -275,6 +329,17 @@ static inline int apic_find_highest_isr(struct kvm_lapic *apic)
> return result;
> }
>
> +static inline int apic_find_highest_isr(struct kvm_lapic *apic)
> +{
> + int vector;
> + if (eoi_enabled(apic->vcpu)) {
> + vector = eoi_get_pending_vector(apic->vcpu);
> + if (vector != -1)
> + return vector;
> + }
> + return __apic_find_highest_isr(apic);
> +}

Why aren't you modifying the ISR unconfitionally?

> +
> static void apic_update_ppr(struct kvm_lapic *apic)
> {
> u32 tpr, isrv, ppr, old_ppr;
> @@ -488,6 +553,8 @@ static void apic_set_eoi(struct kvm_lapic *apic)
> if (vector == -1)
> return;
>
> + if (eoi_enabled(apic->vcpu))
> + eoi_clr_pending_vector(apic->vcpu);
> apic_clear_vector(vector, apic->regs + APIC_ISR);
> apic_update_ppr(apic);
>
> @@ -1236,11 +1303,25 @@ int kvm_get_apic_interrupt(struct kvm_vcpu *vcpu)
> {
> int vector = kvm_apic_has_interrupt(vcpu);
> struct kvm_lapic *apic = vcpu->arch.apic;
> + bool set_isr = true;
>
> if (vector == -1)
> return -1;
>
> - apic_set_vector(vector, apic->regs + APIC_ISR);
> + if (eoi_enabled(vcpu)) {
> + /* Anything pending? If yes disable eoi optimization. */
> + if (unlikely(apic_find_highest_isr(apic) >= 0)) {
> + int v = eoi_clr_pending_vector(vcpu);

ISR != pending, that's IRR. If ISR vector has lower priority than the
new vector, then we don't need to disable eoi avoidance.

> + if (v != -1)
> + apic_set_vector(v, apic->regs + APIC_ISR);
> + } else {
> + eoi_set_pending_vector(vcpu, vector);
> + set_isr = false;

Weird. Just set it normally. Remember that reading the ISR needs to
return the correct value.

We need to process the avoided EOI before any APIC read/writes, to be
sure the guest sees the updated values. Same for IOAPIC, EOI affects
remote_irr. That may been we need to sample it after every exit, or
perhaps disable the feature for level-triggered interrupts.

> + }
> + }
> +
> + if (set_isr)
> + apic_set_vector(vector, apic->regs + APIC_ISR);
> apic_update_ppr(apic);
> apic_clear_irr(vector, apic);
> return vector;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>

--
error compiling committee.c: too many arguments to function